CONVENTIONAL PRESSURE MONITORING
It is long known that peripheral blood pressure (BP) may be estimated using a sphygmomanometer and stethoscope. In this case, when the cuff pressure is between the systolic and diastolic pressures, a sound, called a Korotkoff sound, is heard. By determining the cuff pressure at which sounds are audible through a stethoscope, both systolic (SP) and diastolic (DP) pressures may be estimated. It has been found that the blood pressures so obtained correlate with various physiologic conditions and have both diagnostic and prognostic value. However, using standard techniques, errors in blood pressure determination may occur. These errors are especially common when defining diastolic pressure.
In a manual method of measuring a patient's blood pressure in non-invasive manner, a cuff is applied to an arm of the patient and pumped up to a pressure above the systolic blood pressure of the patient. The arteries of the patient are thereby pressed together in an occluding manner. The cuff pressure is then continuously decreased while the physician or the nurse monitors by means of a stethoscope the start and the end of the opening of the arteries in order to determine on the basis of these so-called Korotkoff sounds: the upper, systolic and the lower, diastolic blood pressure by simultaneously reading these values off from a manometer.
There are also automatic methods for performing this measurement, called "auscultation techniques". The blood pressure monitors employing this technique are not deemed reliable, and in fact are subject to errors and artifacts. In addition, often these techniques produce a result which fails to reveal useful clinical information. One such device is disclosed in U.S. Pat. No. 5,509,423.
Blood pressure monitors and blood pressure measuring methods, respectively, have been employed for a number of years in which the so-called oscillometric methods are utilized, which employ the oscillations or fluctuations of the walls of the arteries which occur in synchronism with the blood pulse. According to the oscillometric techniques, a cuff is pumped up to a pressure beyond the systolic pressure and is then deflated in discrete steps. Alternatively, a cuff is inflated in discrete pressure steps up to a predetermined measure beyond the systolic pressure. There is no universally accepted scheme for measuring blood pressure using oscillographic methods; however there are a number of commonalties in the various proprietary techniques.
During each step, where the cuff pressure is held substantially constant (to avoid artifacts), see, e.g., U.S. Pat. Nos. 4,349,034, and 4,074,711 and European Patent Nos. EP-A-208520, EP-A-353315, and EP-A-353316, or continuously inflated or deflated, see, e.g., U.S. Pat. No. 4,625,277 and European Patent Nos. EP-A-249243 and EP-A-379996, a pressure sensor detects the oscillations caused by movement of the arterial walls and superimposed on the cuff pressure. The amplitudes of these oscillations are recorded. It is thought by many that the oscillations, at the systolic or diastolic pressure, respectively, have an amplitude value or peak-to-peak value that is a fixed percentage of the maximum amplitude or maximum peak-to-peak value at mean pressure. Other criteria for translating oscillometric waveform data into blood pressure are known, and employed in the art. Thus, in the oscillometric measuring method the pressure determined as systolic or diastolic pressure generally is the pressure at which the amplitude or peak-to-peak value of the oscillations is at a specific cutoff, e.g., a percentage of the maximum amplitude of the oscillations.
These various oscillographic blood pressure measurements are prone to artifacts. Typical disturbances superimposed on the pressure signal are movements of the patient and muscular tremor such as shivering. In addition, there are physiological peculiarities, including arrhythmias, such as bigeminy and trigeminy, as well as the cyclic changes of BP due to respiratory variation. In the case of respiratory variations, these changes are real, and may themselves have diagnostic significance.
Oscillometric blood pressure monitors may selectively disregard oscillations, which are related to artifacts. An artifact in known blood pressure monitors is recognized on the basis of a criterion derived from the so-called oscillation channel. In oscillometric blood pressure monitors, the oscillation channel is understood to be a signal channel obtained on the basis of the so-called pressure channel signal, which constitutes the pressure sensor output, by high-pass filtering. This oscillation channel thus corresponds to the harmonic waves or oscillations superimposed on the pressure channel, disregarding the constant component. According to some known systems, this oscillation channel signal is rejected as having a superimposed artifact when either the ascending slope of an oscillation exceeds a maximum increase value or when, at a pressure step, the amplitude difference of two adjacent oscillations exceeds a maximum value or when an envelope criterion is not fulfilled according to which an examination is made as to whether two oscillation amplitudes have not become more than double or less than half between two adjacent steps or when the time interval between two oscillations varies by more than a specific percentage of the average time interval. Such a system, however, is not capable of making a distinction between movement artifact, cardiac arrhythmia or respiratory superimposition. U.S. Pat. No. 5,355,890, incorporated herein by reference, relates to a system for oscillographic blood pressure measurement, employing pulse extraction techniques.
Because of the susceptibiity of the algorithm used in the known oscillometric blood pressure monitor, both erroneous measurements and unnecessary alarms occur. This is of significance in particular since such blood pressure monitors are often employed in operating rooms where a multiplicity of other parameters of a patient must also be monitored, which may all cause alarms. Such medical apparatus must therefore keep the number of false alarms as low as possible, however without risking the recognition of a genuine physiological alarm.
U.S. Pat. No. 5,222,020 describes a blood pressure measuring apparatus which is coupled with an occlusive cuff in order to acquire dynamics on a pulsatile wall motion of human artery responding to the occlusive cuff as its pressure is lowered. The instantaneous cuff pressure (Pc) is first obtained with a pressure transducer; then its value is displayed on a CRT in real time as height variations of a mercury manometer along with the dynamic parameters describing the pulsatile wall motion. The dynamic parameters are basically its displacement velocity and acceleration of the motion generated by blood flow pulsating against the lowering Pc, which reflects the mechanical cardiac cycle of heart as reported by F. Takeda, et al., in Med. Bio. Eng. Comput., Vol. 29, Supplement Part 1, 1991 which is hereby incorporated by reference. See, M. Borow et al., Am. Heart J., Vol. 103, 1982; U.S. Pat. Nos. 4,718,428, 4,796,184, and 4,793,360.
U.S. Pat. No. 5,178,154, incorporated herein by reference, relates to an impedance plethysmographic method utilizing peak aligned ensemble averaging. U.S. Pat. Nos. 5,379,774 and 5,297,556, incorporated herein by reference, relate to impedance plethysmographs which measure arterial elasticity by changes in arterial volume. U.S. Pat. No. 5,331,968 relates to an inductive plethysmographic transducer.
U.S. Pat. Nos. 5,409,009 and 5,391,190 relate to implanted impedance plethysmography devices for use in association with pacemakers. U.S. Pat. No. 5,188,106 relates to an implanted ultrasound transducer for measuring cardiac output and controlling a pacemaker. U.S. Pat. Nos. 5,496,361 and 5,480,412 relate to cardiac wall accelerometers for control of a pacemaker.
U.S. Pat. No. 5,370,122 relates to a cardiac monitoring device.
DEVICES THAT MEASURE PVR
There are a number of available devices that non-invasively measure Cardiac Output (CO). They use a variety of technologies. Each of these technologies determines peripheral vascular resistance as a function of a determined flow and pressure. Thermodilution is an invasive procedure that carries a risk of mortality and is expensive. See, U.S. Pat. No. 5,241,966, incorporated herein by reference. Transthoracic Impedance monitors are difficult to use and do not provide accurate information. On the other hand, they are noninvasive and carry no risk. U.S. Pat. No. 5,309,917 relates to a system for impedance plethysmography, a technique for noninvasive cardiac monitoring. Echocardiography is also noninvasive, but is expensive, relatively inaccurate and requires a skilled technician.
U.S. Pat. No. 5,390,679, incorporated herein by reference, relates to a cardiac output determining device which senses an arterial pressure waveform and compares the sensed waveform to a plurality of stored waveforms representative of known states.
U.S. Pat. No. 5,265,615, incorporated herein by reference, relates to a method for measuring systemic vascular resistance based on an analysis of pressure waveforms including a first dichrotic notch.
U.S. Pat. No. 5,211,177, incorporated herein by reference, relates to a non-invasive vascular impedance measurement system using a modified Windkessel model of the arterial system.
WIDEBAND EXTERNAL PULSE MONITORING
When using the standard auscultatory BP measurement technique, only a very small percentage (approximately 10%) of the energy recorded is within the audible range. Thus, the majority of the energy is dissipated as low frequency signals. These signals can be detected using appropriate wideband transducers. Surprisingly, when using such transducers, signals can be recorded when the BP cuff is inflated above SP.
Description of WEP signal
When a bolus of blood is ejected from the left ventricle, by a heart beat, a (pulse) wave of energy is created which travels from the heart to the periphery of the arterial system. When the energy wave comes up against a barrier (in this case where the arteries become very tiny arterioles), the wave is reflected back into the circulation, traveling from the periphery back towards the heart and great vessels. The majority of the energy in the pulse wave reflection is in the low frequency range. Both forward and backward waves can be recorded using a wideband low frequency transducer placed over the brachial artery.
Wideband external pulse (WEP) recording is based on the ability of a pressure sensor to record inaudible frequencies (down to 0.1 Hz) during blood pressure cuff deflation. Three distinct components of the WEP signal can be detected, called K1, K2 and K3.
The K1 Signal
With cuff pressure above SP (at a point when no Korotkoff sounds are audible), a low frequency signal (K1) is present. For each individual, K1 has a characteristic shape generally consisting of 2 systolic peaks and 2 troughs. The second trough represents the separation of the systolic and diastolic portions of K1. The early peak represents the energy generated by the contraction of the heart as the pulse wave travels from the heart toward the periphery. The early systolic K1 pattern is determined by ventricular ejection (stroke volume) and large artery stiffness.
The second (late) systolic K1 peak represents a measure of arterial pulse wave reflection. Wave reflection in the arterial system occurs from arterial terminations i.e. the arteriolar bed. Peripheral vascular resistance is a measure of the degree of contraction of the arteriolar bed. Since the level of vasoconstriction of the arteriolar bed is the major factor for both peripheral vascular resistance ("PVR") and the intensity of pulse wave reflection, the K1 pattern varies with measure peripheral vascular resistance. Other factors, such as age (i.e. arterial stiffness) may be involved in the baseline K1 pattern, but acute changes are due to changes in PVR.
K1 Analysis--Description of K1R
Three vectors are defined from baseline: the initial peak (Y1), the subsequent trough (Y2), and the second systolic peak (Y3), as shown in FIGS. 9A and 9B. FIG. 9A shows a typical K1 pattern of a young person with normal blood pressure, while FIG. 9B shows a typical K1 pattern of an elderly hypertensive patient.
These patterns (K1 pattern) are reproducible in individuals, tend to change with age, yet have been found to vary in different physiological states. Analysis of these waves has led to a derivative called the K1Ratio and the related K1R.
A K1 Ratio is calculated by: EQU K1 Ratio=(Y1-Y2)/Y3 EQU K1 R=in (K1 Ratio)
Thus, K1 R is the natural log of the K1 Ratio.
It has been demonstrated that this ratio declines with age, but more importantly, can change many-fold in a particular individual depending upon the state of vasodilatation. Thus, the concept has been developed that changes in K1R (and the K1 Ratio) are due to changes in reflectance of waves in the circulation. As such, K1R can be used as a direct measure of both the physical properties of large arteries and the degree of peripheral vasomotor tone.
The K2 Signal
K2 appears at SP and disappears at diastolic pressure, which approximately corresponds to the audible Korotkoff sound. The appearance/disappearance property of K2 is the basis for an objective and more accurate method for measuring blood pressure, called K2 analysis. A legitimate Korotkoff sound cannot be present without the visual presence of K2.
K2 Analysis
The K2 analysis technique using Wideband External Pulse (WEP) recording correlates better with the intraarterial blood pressure than the auscultatory technique. Blank, S. et al., Circulation, 77:1297-1305,1988. See also, Blank, Seymour G., "The Korotkoff Signal and its Relationship to the Arterial Pressure Pulse", Ph.D. Thesis, Cornell University (1987) (UMI 8810638), expressly incorporated herein by reference.
The presentation of WEP data in more than one dimension has been the subject of some study. Denby, L. et al., "Analysis of the Wideband External Pulse: An application of Graphical Methods", Statistics in Medicine, 13:275-291,1994.
There are situations in which the auscultatory technique has acknowledged difficulty. These include the presence of auscultatory gaps, pregnancy, and narrow pulse pressures.
WEP measurements have been proposed to assist in the interpretation of peripheral blood pressure measurements in the presence of auscultatory gaps. Blank, S. et al., "Characterization of Auscultatory Gaps With Wideband External Pulse Recording", Hypertension, 17(2):225-233, 1991.
In pregnancy and narrow pulse pressures, WEP measurements have been used as a validation standard with which to evaluate the auscultatory technique. Blank, S. et al., "How should diastolic blood pressure be defined during pregnancy?", Hypertension, 24:234-240,1994. Blank, S. et al., "Isolated elevation of diastolic blood pressure: real or artifactual?" Hypertension, 26:383-389, 1995. WEP has also been employed to assess underdeveloped K2 (auscultatory gaps) with respect to vascular stiffness and atherosclerosis. See, Cavallini et al., "Association of the Auscultatory gap with Vascular Disease in Hypertensive Patients", Ann. Intern. Med. 124:877-883 (1996).
The K3 Signal
K3 appears with cuff pressure between SP and DP and continues to be present below DP. K3 resembles the intraarterial pressure waveform. Thus, when calibrated according to K2 analysis (i.e. SP and DP), direct determinations of mean arterial pressure and noninvasive dp/dt measurements can be made.
Measurement of Mean Arterial Pressure from WEP Recording
The determination of mean arterial pressure is traditionally based on the formula: EQU MAP=Diastolic Pressure (DP)+k.times.(SP-DP)
where k represents a form factor which is generally assumed to be 1/3. In actuality, k depends on the shape of the intraarterial pressure pulse, and can vary from 0.2 to 0.5. Thus, significant errors can occur when calculating MAP in the traditional manner (from SP, DP and k factor).
Using WEP Recording, DP and SP can be accurately determined using K2 Analysis. Since K3 closely resembles the intraarterial pulse, and can be calibrated according to analysis of K2, MAP can be directly measured from the area under the curve. Analysis of K3 can yield an accurate measure of the k factor mentioned above.
Physiological Studies Relating K1 Ratio to Peripheral Vascular Resistance
In 12 elderly patients, immediately prior to undergoing major joint replacement surgery, measurements of K1Ratio (and K1 R), cardiac output (CO), peripheral vascular resistance (PVR) and other hemodynamic variables were concurrently measured during 5 different physiological states. These included infusions of epinephrine (E) and norepinephrine (NE) both before and following epidural blockade. The results of this study were published in 1994 ("Comparison of Changes in K1 ratio and Systemic Vascular Resistance following Epidural Anesthesia as indices of Vasodilatation", ASRA Annual Meeting 1994, p. 69).
Assessment of Cardiac Contractility Using WEP Recording
A measure of cardiac contractility can be determined noninvasively by determining the rate of rise of a calibrated K3 signal using the so-called dp/dt concept. Similarly, a measure of cardiac contractility may be derived from the upstroke of a calibrated K1 pattern.
Systems for Measurement of Wideband External Pulse
According to the prior art, the system designed to measure wideband external pulse (WEP) acoustic emissions employs high precision, large dynamic range foil electret microphone with a linear high impedance electrometer.
Various piezoelectric materials are known, which are able to convert vibrations or movements into electrical impulses. These may include polyvinylidene fluoride polymers, e.g., Kynar.RTM., or polylactic acid. See, U.S. Pat. No. 5,298,602. AT&T provides a type of wideband Foil Electret Sensor, with no significant change in sensitivity under a pressure range of at least 0 to 250 mm Hg, with sensitivity over its entire surface and a flat (-3 dB) bandwidth of from below 0.1 Hz to above 2000 Hz. Therefore, such a Foil Electret microphone may be used as a wideband acoustic transducer in an apparatus to obtain the wideband external pulse, connected to a high impedance (10.sup.9 .OMEGA.) amplifier, such as a Keithly electrometer (Model 600B) (Keithly Instruments, Cleveland Ohio) and then to a direct current amplifier model DCV-20 of an Electronic for Medicine/Honeywell model VR6 physiologic recording system (Electronics for Medicine, Pleasantville, N.Y.).
The known device includes an inflatable cuff for encircling the arm and receiving vibrational signals over the brachial artery. The cuff pressure may be controlled by a Hokanson E-10 cuff inflator (Hokanson, Issaquah Wash.) and the pressure may be manually read with a mercury column or a Gould-Stratham P23 ID or T4812 AD-R (Gould-Stratham, Oxnard, Calif.) pressure transducer connected to the physiologic recording system through a PDV-20 amplifier. The deflation rate of the Hokanson unit is manually set to about 2-4 mm Hg./sec.
The wideband acoustic data may be analyzed with a computer system, such as a DEC LSI 11/23 computer, sampling at 400 samples per second with 12 bit resolution. An IBM PC/AT or equivalent may also be used, sampling a 12 bit analog to digital converter at 500 samples per second, using CODAS (Dataq, Akron Ohio) data acquisition software.
Other Transducer Systems
An electret transducer array, as disclosed in U.S. Pat. No. 5,388,163, incorporated herein by reference, may be constructed of an electret foil and a backplate. The electret foil is flexible, having two layers, a metal (such as aluminum) layer and a synthetic polymer (such as PTFE Teflon.RTM.) layer. The metal layer may be, e.g., two thousand Angstroms thick, while the polymer layer may be, e.g., between 2-100 microns thick. The polymer layer is given a permanent charge, to form an electret, to a predetermined value at, e.g., -300 volts, by conventional techniques. A positive compensating charge is induced in the backplate and the metal foil layer.
The electret element is situated to be sensitive to the acoustic waves traveling in the tissue. Thus, a mounting is provided which provides a vibration-free reference. Thus, any piezoelectric activity in the electret element is presumed due to relevant acoustic waves and not artifact. Thus, the transducer is used to detect vibrations from the brachial artery through skin and tissue. A backplate may be formed of a sintered or porous material to allow air flow behind the element while providing structural rigidity.
Multiple segments of an electret transducer array may be formed by the selective removal of the metal layer from the electret foil to achieve transducers of any desired shape, size, and location. Selective removal of portions of the metal foil layer for the purpose of forming segments may be accomplished by etching or dissolving the metal using a chemical reagent, such as a solution sodium hydroxide or ferric chloride, or otherwise in known manner with a variety of chemical and/or photoetching treatments.
Alternatively, segments may be defined on the foil prior to charging and mounting on the backplate. This may be done by selectively metalizing the polymer layer to form a foil. Selective metalization may be performed by conventional metal deposition techniques (e.g., masking, evaporation, sputtering, etc.) to form segments of any desired size, shape, and location. A continuous electrode foil having a polymer layer selectively charged (with either or both polarities) in defined locations may also be used.
Electrical leads are coupled to each individual electrode segment. Also provided is an electrical lead, coupled to the backplate, which may serve as a common lead for the transducers of the array. The electrical leads for the segments may also be formed as conductive traces on the surface of the electret element, preferably electrically insulated from the surface. By means of these leads, electrical signals produced by each transducer in response to incident acoustic signals may be accessed for amplification and other processing.
An alternative piezoelectric transducer may be used as a hydrophone, as disclosed in U.S. Pat. No. 5,339,290. Typical suitable polymers include PVDF, but the copolymer P(VDF-TrFE) is preferred because of its flexibility with regard to the poling process that is conventionally employed in defining a piezoelectrically strong active area. For example, the active area may be provided at the center of the piezoelectric membrane, which may be a single-sheet type or bilaminate. U.S. Pat. No. 5,365,937 relates to a piezoelectric transducer for receiving heart sounds. U.S. Pat. Nos. 5,337,752 and 5,301,679 relate to systems for the analysis of body sounds.
As disclosed in U.S. Pat. No. 5,363,344, a transducer may be formed of a material called C-TAPE by C-TAPE Developments, Ltd., 3050 S. W. 14th Place, Boynton Beach, Fla. 33435. This material is the subject of U.S. Pat. No. 4,389,580, hereby incorporated by reference.
Therefore, the prior art discloses systems capable of obtaining wideband external pulse ("WEP") signals under laboratory conditions, and further discloses studies analyzing data so obtained to determine blood pressure. The prior art acknowledges the richness of the information included in the WEP signals, but does not teach or suggest how this information may be extracted and employed to determine the cardiac status of an individual patient, other than blood pressure, and further does not disclose automated instruments for obtaining and analyzing the WEP data. Therefore, the prior research of the present inventor remains inaccessible in a clinical setting.